Try adding hot water after a fill to see if the unit will then run OK.
Just be careful to not overfill it.

If the unit runs OK then you know it is a fill problem.

Shut the water off and disconnect the unit's fill valve.
Check the filter screen on the input side of the valve.
If it is dirty clean it.
Do not permanently remove it as it is there to keep crud out of the valve which could cause a flood.

My thinking above is that there is crud on the walls of the water line and some may have come loose during installation and is now clogging the filter.
If it is dirty bang on the supply line and run some water through it before reinstalling the fill valve.

If the screen is clean then I would replace the shut off valve.
Could be thara is a problem with it.